This invention pertains to the manufacture of integrated circuits and, more particularly, to the use of a laser for trimming elements of the circuit.
During the manufacture of integrated circuits, it is a common practice to deposit circuit elements, such as resistors, in the form of thin films slightly larger in area than ultimately required. Thereafter, the element is carefully trimmed by removing portions thereof in response to the results of electrical measurements. In this way the element may be trimmed to a desired value within the tolerances established by the designer. In the case of a resistive element, for example, trimming is continued until the resistance is raised to the desired value.
One commonly employed technique for such trimming is by use of a laser. The laser beam is carefully focused onto the circuit element, which usually lies below an oxide, or other transparent dielectric, layer. The laser wavelength selected is such as to be absorbed by the circuit element, which is thereby heated and vaporized.
This technique, while successful and widely used, has certain limitations. For example, laser-trimmed elements frequently exhibit slight but undesirable instability effects which limit the trim tolerance, and thus also the performance level, of the device. It has been found that such instability may be caused by a characteristic of the laser beam in that the energy of the beam is not constant across its diameter. As might be expected, the energy varies in the form of the well-known bell, or Gaussian, curve. The greatest energy is typically concentrated at the optical axis of the laser beam. The beam impinging upon the circuit element volatilizes the material of the element from the optical axis outwardly radially to some dimension corresponding to the "volatilization threshold" of the material. Beyond this threshold radius, outward, to the beam edge, volatilization does not occur. However, that material, so exposed to this low power periphery of laser energy, has changed electrical properties (e.g., resistivity), which are thermally unstable. For each circuit element, this instability is variably dependent on the extent of laser trim of such element. Subsequent manufacturing (i.e., packaging of the device) or other adverse environmental conditions (i.e., long term stability burn-in) will induce a small but unique change in electrical resistance of such circuit element.
The accuracy of performance of a device with a plurality of resistive elements may depend upon the resistive circuit elements remaining fixed in resistive values relative to one another. The designed performance of the device is limited by the above-mentioned induced change in resistive value of each element, as determined by the extent of laser trimming performed on each such element.
For the foregoing reasons, it would be desirable to provide a method for laser trimming a circuit element whereby instability effects are eliminated or substantially minimized. To that end, in accordance with the invention a procedure is provided wherein a clean kerf is formed and the edge of the remaining circuit element is protected from those portions of the laser beam which are below the vaporizing threshold level. The manner in which this and other objects are achieved will be apparent from the following description and appended claims.